One technique employed in packaging the electronic circuitry of digital computers, particularly in large scale computer systems, is the use of micropackages, each of which contains a large number of integrated circuits interconnected to perform a given function, the micropackages, in turn, being appropriately interconnected. Each micropackage, of course, is subjected to a regimen of functional tests before it is incorporated into a computer system. Nonetheless, even when these functional tests (and other tests) have been successfully passed, a given micropackage may still later fail in operation, and the failure may be of the elusive "intermittent" type or even a singular failure) which may seem to have no apparent cause other than the possibility of the disruption caused by a stray alpha particle or some other purely transient condition which is not the "fault" of the micropackage. However, it is possible that the fault does, indeed, lie within the micropackage itself despite its successful completion of numerous tests and subsequent "good" record of operation in the field. For example, in current-mode logic or emitter coupled logic, it is possible that one or more integrated circuits in a given micropackage may have in its structure an undetected N+ diffusion "pipe" or "spike" which subtly alters the operating characteristics of the specific transistor in the integrated circuit and may subject it to long term deterioration (often without complete failure) in performance.
N+ diffusion pipes are faults in the base extending from emitter to collector of an integrated circuit transistor as a thin plane. Spikes are similar base faults which do not completely connect the emitter and collector. In general, any such fault will increase the number of recombination sites which decreases minority carrier lifetime. As a result, recombination current in the base region increases which increases base current leading to a decrease in .beta. (i.e., current gain: I.sub.c /I.sub.b). Other, related deleterious effects may arise as a result of the presence of N+ diffusion faults in an integrated circuit.
Thus, it is to the detection of the presence of N+ diffusion faults in a micropackage and to the use of such information in analyzing the "risk" level of a given micropackage to which the present invention is directed.